The present application claims priority to Canadian Application No. 2,364,151, filed with the Canadian Intellectual Property Office on Nov. 29, 2001.
The invention relates to the stimulation of oil and gas wells to improve production and, in particular, to a well stimulation tool used to deliver high-pressure fluids through a production tubing of a well in order to acidize and/or fracture subterranean formations with which the well communicates.
It is well known that many oil and gas wells require stimulation in order to increase production either as a part of well completion, or as a part of well workover. Well stimulation involves the pumping of fluids under high pressure into the annulus of the well in order to break up subterranean formations and release hydrocarbons into the wellbore, where they can be extracted to the surface. Since it is generally not economically feasible to manufacture wellhead equipment that can withstand extreme pressures, the wellhead must be isolated or removed during well stimulation in order to prevent potential damage and/or injury.
One method of stimulating a well is to pump high-pressure fluids down a production tubing suspended in the well. To accomplish this, the production tubing is plugged and the wellhead is removed from the well. An extension is then screwed into the top of the tubing hanger and high-pressure fluids are pumped through a high-pressure valve that communicates with a top of the extension. A high-pressure adapter of this type is described in U.S. Pat. No. 5,103,900 that issued on Apr. 14, 1992 to McLeod et al., and entitled HIGH PRESSURE ADAPTER FOR WELL-HEADS. McLeod et al. describe an improvement to the McEvoy PST adapter and pack-off nipple in which vent ports are added to the McEvoy tubing head adapter to comply with American Petroleum Institute (API) regulations respecting elevations in pressure rating between adjacent adapters.
The McEvoy PST adapter and McLeod et al.""s improvements to it suffer from several disadvantages, however. First, each adapter must be constructed for a specific size of production tubing. Consequently, at least one adapter must be kept in stock for each size of production tubing that is to be serviced. Second, the flow path of high-pressure fluids is interrupted by an internal bore in the tubing head adapter. The internal bore provides a space where eddy currents develop in the high-pressure fluids. The eddy currents tend to cause abrasive well stimulation fluids to xe2x80x9cwash outxe2x80x9d a top end of a pack-off nipple that connects to the tubing hanger. As is well understood in the art, damage caused by wash out can cause dangerous pressure leaks.
There therefore exists a need for a well stimulation tool that permits high-pressure fluids to be safely pumped down the production tubing of a hydrocarbon well.
It is therefore an object of the invention to provide a well stimulation tool that permits high-pressure fluids to be safely pumped down the production tubing of a hydrocarbon well.
It is a further object of the invention to provide a well stimulation tool that is quickly and easily mounted to an exposed tubing hanger of a hydrocarbon well.
The invention therefore provides a well stimulation tool for stimulating a well having a tubing head mounted to a casing spool of the well and a production tubing suspended from a tubing hanger in the tubing head. The well stimulation tool comprises an adapter spool having a bottom flange adapted for connection to one of a tubing head and a blowout preventer mounted to the tubing head. The adapter spool has a top end adapted to receive a mandrel that can be reciprocally moved through a packing that surrounds the mandrel and is retained in a packing cavity in the top end of the adapter spool. The top end of the adapter spool further includes threads for securing a lockdown nut that covers the packing and surrounds an outer periphery of the mandrel.
The mandrel includes a mandrel top end having a flange adapted for the connection of a high-pressure valve, and a mandrel bottom end having a thread for the connection of an adapter pin. The adapter pin has a top end with threads for connecting the adapter pin to the mandrel bottom end. The adapter pin also has a bottom end with threads for connecting the adapter pin to the top end of a tubing hanger.
The well stimulation tool preferably further includes a high-pressure flange adapted to be mounted to a top of the high-pressure valve. The high-pressure flange has a pressure flange bottom end adapted for connection to a top flange of the high-pressure valve. The high-pressure flange also has a top end that is threaded for connection to a high-pressure line for injecting high-pressure well stimulation fluids into the well.
In accordance with one embodiment of the invention, The well stimulation tool further comprises a lifting head assembly including a first member and a second member. The first member has a bottom end adapted to be connected to the high-pressure flange. The second member has means for connecting a hoist to the well stimulation tool. When the lifting head assembly is connected to the combination of the high-pressure flange, high-pressure valve, the adapter spool, the mandrel and the adapter pin, and hoisted above the well, the adapter pin extends below the bottom flange of the adapter pin to facilitate the installation of the well stimulation tool to the tubing hanger.
The lifting head assembly includes a hollow cylinder having a lifting head top end and lifting head bottom end. The lifting head bottom end includes an aperture. A lifting sub having a top end, which reciprocates freely within the hollow cylinder but cannot pass through the aperture in the bottom end, is adapted to connect to a top end of the high-pressure flange. A lifting eye is affixed to the lifting head top end.
The lifting head further comprises opposed, radially extending lift arms adjacent the lifting head bottom end, the lift arms respectively including a lift eye located near an outer end thereof. The lockdown nut also comprises opposed, radially extending lift arms respectively including a lift eye located near an outer end thereof. First and second lifting cables interconnect the lift arms of the lift head and the lockdown nut. Consequently, when the well stimulation tool is lifted using the lifting eye affixed to the top end of the lifting head, the lifting sub top end moves downwardly within the hollow cylinder and the lifting cables bear the weight of the lockdown nut and the adapter spool. As a result, the mandrel is stroked down through the adapter spool and the adapter pin is extended beneath the bottom flange of the adapter spool to permit the adapter pin to be connected to, or disconnected from, the top end of the tubing hanger.
The invention further provides methods for completing and stimulating oil and gas wells.